1. Field of the Invention
This invention relates to optical couplers and connectors for use in interconnecting optical fibers in-line and for coupling optical fibers to circuit boards and the like.
2. Description of the Related Art
Electronic networks, such as those used in data telemetry and voice communication may be interconnected by optical fibers. In certain cases, optical signals may be transferred between other fibers by evanescent couplers. Such couplers transfer a specified portion of the optical energy propagating in one fiber to one or more other fibers.
In an ever-increasing number of applications, optical fibers are being used to transmit signals over substantially large distances. In contrast to traditional wire conductors, optical fibers require special hardware for mating the optical-fiber ends to other fibers, radiation sources such as LEDs, and pulse detectors. One such piece of hardware is an SMA-style connector such as those manufactured by Amphenol in Lisle, Illinois and Ensign-Bickford Optics Company in Avon, Connecticut.
One advantage in using optical fibers is the ability to transmit large amounts of information over strands having diameters substantially equal to human hair without experiencing electromagnetic interference. In contrast, a disadvantage is the requirement that fiber ends be precisely aligned in order to transfer the maximum signal strength. All too often, a portion or all of a signal loss may be due to poorly aligned connections.
Nippon Sheet Glass Company, Limited in Minato-ku, Tokyo, Japan developed a self-focusing microlens for use in coupling optical fibers to each other, optical sources or detectors. The microlens may collimate a diverging beam, converge a collimated beam, or converge a diverging beam, depending upon the length of the lens and the orientation. For the most part, all the microlenses are wave-length dependent.
The microlens is a cylindrical glass or plastic rod having a graded index of refraction which decreases as the square of the radial distance from the optical axis. Because of the parabolic index of refraction, the microlens performs the same optical functions as standard spherical lenses with the added feature of flat end surfaces and no spherical aberration. The cylindrical glass or plastic lens is concentrically mounted in one end of a metal sleeve. An optical fiber cemented within an aligning insert is inserted into the opposite end of the sleeve so that the end of the optical fiber is positioned near the focal point of the lens. The lens may accept light energy being emitted by the fiber end and may concentrate light energy at the focal point in the fiber tip, thus energy transfer between the lens and fiber is maximized.
The microlens described above is used primarily in the laboratory where sensitive and fragile centering guides align the microlenses in end-to-end arrangement. The networks in which they are arranged are so delicate that a gentle brush with the aligning table often results in an interruption of the system. Although the microlens requires a much less precision alignment than standard SMA-type connectors, they are not rugged enough for use in field applications.